The present invention is related to an improved transmission mechanism, and more particularly to a transmission mechanism of drilling/milling tool for controlling the forward and backward travel of the drill bit or blade of the drilling/milling tool.
A conventional drill bit or milling blade is arranged on a table and driven by a motor to rotate or reciprocally move for drilling or milling a work piece. Taiwanese Patent No. 78206140 (Publication No. 146298), No. 79200031 (Publication No. 205746) and No. 83212120 (Publication No. 241592) disclose many typical drilling or milling tools. The conventional drilling tool includes a power drill bit driven by a motor and axially reciprocally movable along the mandrel forward or backward. A reducing device and an induction motor (subsidiary motor) and a rotational mechanism are disposed between the motor and the transmission shaft for transmitting the rotational power of the motor. A transmission mechanism is disposed between the power drill bit and the main body for controlling the drill bit to drivingly couple with the transmission shaft. Accordingly, the drill bit can be rotated and moved forward or backward.
In forward and backward travel of the drill bit or blade, the transmission shaft or mandrel is rotated in reverse directions. Also, the rotational speed of the drill bit or blade in the forward travel is apparently slower than that in the backward travel. This is for preventing the drill bit from damaging the cutting angle of the work piece in the forward operation. In fact, the control of the rotational speed in forward or backward travel is achieved by the reducing device and induction motor. However, it is known by those skilled in this field that the reducing device and induction motor themselves and the peripheral mechanisms cooperating with the reducing device and induction motor are arranged in a complicated state. For example, between the driving gear, driven gear and belt included in the reducing device, at least a wheel shaft having a shaft hole is disposed on two sides of the driven wheel and adapted to the tooth structure of rear section of the main shaft. Accordingly, the driven gear changeably commonly accepts the power transmitted from the driving gear. Therefore, some manufacturers use servomotor which can forward and backward rotate with variable rotational speed instead of the above induction motor and reducing device. This can combine the requirements for operation. However, the cost/price of the servomotor is quite high so that the drilling/milling tool as a whole is quite expensive. This directly affects competition of the product on market.
Representatively speaking, the various types of conventional transmission mechanism applied to the drilling/milling tool fail to achieve both functions of forward and backward rotation and changing rotational speed by means of simple structure and cheaper ordinary motor adapted to the structure.
It is therefore a primary object of the present invention to provide a transmission mechanism of drilling/milling tool, which has small volume and simplified structure applicable to hand tool. In the reciprocal forward and backward travel of the drill bit or blade, the drill bit or blade can be forward or backward rotated at changeable corresponding low or high rotational speed. This eliminates the problems of complicated structure and high cost for the servomotor existing in the conventional device.
According to the above object, the transmission mechanism of drilling/milling tool includes an ordinary motor having a transmission shaft capable of forward and backward rotating. The transmission mechanism is able to transmit the power of the transmission shaft to the drill bit or blade. The transmission mechanism includes a slide gear movable along a longitudinal reference axis and a first and a second driven wheels engaged with the slide gear. The slide gear and the first driven wheel or the second driven wheel has a brake or shaft assembled therewith. The slide gear is responsive to the forward and backward rotation of the transmission shaft to selectively drivingly engage with the first or second driven wheel so as to change the rotational speed at the power output end.
The brake of shaft at least has a threaded section. The slide gear is formed with an inner thread for screwing with the threaded section of the brake of shaft. Within a set range, the slide gear is permitted to axially upward move along the threaded section of the brake or shaft. In the forward travel of the drill bit or blade, only the first drive wheel is driven to create forward rotation at slow speed at the output end. In the backward travel of the drill bit or blade, along with the reverse rotation of the transmission shaft, the small ratio gear of the second driven wheel is selectively driven to create rotation at high speed at the output end.
The present invention can be best understood through the following description and accompanying drawings wherein: